Disaccharides

Question 1

Disaccharides definition

Answer 1

Consist of 2 monosaccharide units joined by an O-glycosidic bond

Question 2

Disaccharide classification

Answer 2

Can be classified into:
1. Homodisaccharides: formed of the same monosaccharide
Ex: maltose, isomaltose, cellobiose
2. Heterodisaccharide: formed of different monosaccharides
Ex: sucrose and lactose
Most abundant: sucrose, lactose, maltose

Question 3

Disaccharides can be also classified into

Answer 3

1. Non reducing disaccharides: the two aldehyde or ketone groups are involved in the linkage (anomeric C not free)
   Example: sucrose
2. Reducing disaccharides: one of aldehyde or ketone groups is free (anomeric C is free)
   Example: lactose and maltose

Question 4

Maltose

Answer 4

Malt sugar, maltose syrup
Formed of (structure): two alpha D-glucose units
Linked by (bond): linked together by alpha 1-4 glycosidic bond
Produced by (source): action of amylase enzymes on starch
Reduction: reducing sugar
-it is hydrolyzed by maltase enzyme to produce 2 glucose molecules

Question 5

Isomaltose

Answer 5

Structure: two alpha glucose
Bond: alpha 1-6 glycosidic bond
Reduction: reducing sugar
Source: obtained by hydrolysis of some polysaccharides such as dextran

Question 6

Cellobiose

Answer 6

Structure: two units of beta glucose
Bond: beta 1-4 glycosidic bond
Reduction: reducing sugar
Source: obtained by the acid hydrolysis of cellulose

Question 7

Sucrose

Answer 7

Cane sugar, beet sugar, table sugar
Structure: alpha D-glucose and beta D-fructose
Bond: alpha-1-beta-2 glycosidic bond (involving the anomeric active carbon atoms)
Reduction: non reducing sugar

-Hydrolysis of sucrose to glucose and fructose is catalyzed by the sucrase (also called invertase)
-sucrose is dextrorotatory
-an equilibrium mixture of glucose and fructose is levorotatory
-this equilibrium mixture is called invert sugar (in bee honey) and has free carbonyl group (reducing sugar)

Question 8

Lactose

Answer 8

Milk sugar
-principle carbohydrate in milk
-critical nutritional importance to mammals in the early stages of their lives
-Strcture: formed from beta galactose and beta glucose
-Bond: beta 1-4 glycosidic bond with free anomeric C
-Reduction: reducing sugar
-appear in urine in late pregnancy and during lactation
-hydrolyzed by intestinal lactase enzyme into galactose and glucose

Question 9

Clinical correlates to lactose

Answer 9

Low levels of lactase enzyme lead to undigested lactose that undergoes bacterial fermentation in the colon with the generation of large amounts of CO2, H2 and irritating organic acids

These products cause painful digestive upsets known as lactose intolerance